A boosted engine may exhibit higher combustion and exhaust temperatures than a naturally aspirated engine of similar output power. Such higher temperatures may cause increased nitrogen-oxide (NOX) emissions from the engine and may accelerate materials ageing, including exhaust-aftertreatment catalyst ageing. Exhaust-gas recirculation (EGR) is one approach for combating these effects. EGR works by diluting the intake air charge with exhaust gas, thereby reducing its oxygen content. When the resulting air-exhaust mixture is used in place of ordinary air to support combustion in the engine, lower combustion and exhaust temperatures result. EGR may also improve fuel economy in gasoline engines by reducing throttling losses and heat rejection.
In boosted engine systems equipped with a turbocharger compressor mechanically coupled to a turbine, exhaust gas may be recirculated through a high pressure (HP) EGR loop or through a low-pressure (LP) EGR loop. In the HP EGR loop, the exhaust gas is taken from upstream of the turbine and is mixed with the intake air downstream of the compressor. In an LP EGR loop, the exhaust gas is taken from downstream of the turbine and is mixed with the intake air upstream of the compressor.
HP and LP EGR strategies achieve optimum efficacy in different regions of the engine load-speed map. For example, on boosted gasoline engines running stoichiometric air-to-fuel ratios, HP EGR is desirable at low loads, where intake vacuum provides ample flow potential; LP EGR is desirable at higher loads, where the LP EGR loop provides the greater flow potential. Various other tradeoffs between the two strategies exist as well, both for gasoline and diesel engines. Such complementarity has motivated engine designers to consider redundant EGR systems having both an HP EGR loop and an LP EGR loop. However, fully redundant HP and LP EGR systems can be heavy and expensive—each loop including conduits, heat-exchangers, control valves, and in some cases, flow sensors. Further, fully redundant HP and LP EGR systems are typically unable to route exhaust gas from an HP take-off point to an LP mixing point, as may be desired under some operating conditions.
Therefore, the inventor herein has provided an integrated HP and LP EGR system for a boosted gasoline or diesel engine, in which certain cost-, weight-, and package-intensive components are shared between the two loops. In one embodiment, a system for inducting air into an engine is provided. The system includes a compressor and a turbine mechanically coupled to the compressor and driven by expanding engine exhaust. The system also includes a first conduit network configured to route some engine exhaust from a take-off point downstream of the turbine to a mixing point upstream of the compressor, and, a second conduit network configured to route some engine exhaust from a take-off point upstream of the turbine to a mixing point downstream of the compressor. The first and second conduit networks in the system have a shared conduit and a control valve configured to adjust an amount of engine exhaust flowing through the first conduit network and to adjust an amount of engine exhaust flowing through the second conduit network. The system also includes a flow sensor coupled in the shared conduit.
In this way, HP and LP EGR are provided in the same engine system, but without incurring the full cost, weight and packaging complexities of a fully redundant, two-loop EGR system. Moreover, the disclosed system allows EGR to be routed from an HP take-off point to an LP mixing point. Such functionality may be useful in averting compressor surge and increasing EGR flow potential under certain operating conditions.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.